High frequency transmitter



, Sept 4 1934- F. P. BEST y 1.972,370

HIGH FREQUENCY TRANSMITTER Filed Feb. 25. 1933 INVENTOR l FHM/r Aflou/fa mr ATTORNEY Patented Sept. 4, 1934 HIGH FREQUENCY TRANSDIITTER Frank Powell Best,l lion-don, England, assignor to Radio Corporation of America, a corporation oi' Delaware .l Application February 25, 1933, serial No. 658,621

- InA Great BritainMarcln 4, 1932 claims. (crass- 17) This invention relates to radio and otherhigh frequency transmitters and more particularly to high frequency telegraph transmittersoffthe soc'alled absorber type by vwhich expression is meant transmitters of the type wherein the load upon the driving source of anode'potential is maintained substantially constant during all conditions of keying by providingatube' in shunt across said source, said tube being controlled in accordance with the keying so that -it places upon said source a load of such magnitude'gthat the total load thereon (which is madefup of the load due to the absorber tubefan'd thatdue to the signalling tube' or tubes) is constant during keying. i y f 'I'he invention has for its object to provide an absorber type transmitter which shall be simple and cheap to construct, easy to control, simple as regards keying arrangements andcapable' of utilizing alternating current of a frequency usually available (e. g. 500 cycles or 50 cycles single phase or multiphase) forkeying. Further objects are to provide a transmitter in which all the parts except of course the key itself (or keying relay if any) shall be static, i. e. shall not involve the use of mechanically moving parts, and wherein key-clicks shall be entirely eliminated or substantially reduced.

According to this invention keying is accomplished in a transmitter of the kind in question by controlling the alternating current supply to a rectifier unit whose rectified output is utilized to provide bias potentials to the absorber tube er tubes and to the oscillating tube or tubes the arrangement being such that the total load upon the source of anode potential is substantially the same for both positions of the keying means.

The invention is illustrated in the diagrammatic drawing accompanying the provisional specification which drawing shows one arrangement in accordance with the invention. Referring to the said drawing, 1 is a high frequency oscillating tube whose output is coupled to a transmitting aerial as shown. The anode potential for said tube is supplied between the terminals HT+ and HT- and in shunt across these terminals is connected the anode-cathode space of an absorber tube 2 having the usual resistance 3 in its anode circuit. Controlling bias for the tubes 1, 2 is obtained from tappings on a potentiometer resistance 10 which is shunted by a condenser 11, said potentiometer resistance being connected to receive the rectified output from a full wave rectifier which in the example illustrated comprises a pair of diodes 4, 5. Obviously,

' however, any other suitable rectifier arrangement 'may be employed e. g. a rectifier having a metallic-envelope which acts as an electrode may repla'ce the diodes. Alternating current is fed to the diodes or other rectifier arrangement through -a transformer '7 and a variable resistance 8 in series with said transformer 'l serves toadjust l the potential applied to the primary of said transformer. Of course, additional direct currentbias may beapplied to the grid of any ofthe tubes (e.-g.`the tube 2) by means of a battery or other is a key 9 the keying circuit being energized from f an available' alternating current source which is indicated by the letters AC and may be of the customary 800' cycle frequency commonly` available for shipboard and similar transmitters. The .alternating current'source also supplies filament -heating 'current for the various tubes as shown via transformers, 4the transformer 6 serving to energize the filaments of the diodesfi and 5. Of course where a suitable direct current supply is available the two tubes l, 2 may have their filaments energized by direct current instead of, as shown, by alternating current. n

The circles shown in the drawing indicate the usual measuring instruments for observing the performance of the arrangement as a whole. When the key 9 is closed a voltage drop will be set up across the resistance 10 a point on which is, as will be seen, earthed. As a result, when the key is closed the grid of the tube 2 receives positive potential and the grid of the tube 1 negative potential so that in these circumstances the tube 2 passes current and applies a predetermined load across the terminals HT| and IIT-, while the tube 1 is prevented from oscillating. The load applied by the valve 2 in these circumstances is made equal to the load applied across the same terminals by the tube 1 when the said tube is oscillating.

When the key 9 is open the grid of the tube 2 and the grid of the tube 1 are both connected to earth through the appropriate parts of the resistance 10 and if the tube 2 be of suitable characteristics no current will flow therethrough in these circumstances, while the tube 1 will generate high frequency oscillations. By adjusting the positions of the tapping points upon resistance 10 the total load upon the terminals HT+ and HT- may be maintained constant While keying. A further adjustment by means of the resistance 3 enables accurate control of the voltage set up A.IDO

across the resistance 10 to be obtained and this makes the regulation of the current absorbed by the absorber tube Very simple. The condenser 11 of course acts as a smoothing system and controls the rate at which voltage is built up across the resistance 10. This condenser, which may if desired be replaced by a more complex smoothing system, should be adjusted to reduce key-clicks to a minimum.

In an actual embodiment the maximum voltage output from the rectifying system was about 1,000 volts and the current in the resistance 10 about 100 milli-amperes. The condenser 11 was about .25 microfarads capacity and the absorber tube shown in the drawing as one tube consisted in fact of two tubes in parallel.

It is found that by accurately adjusting the various components of the circuit it is possible practically to reduce key-clicks to such an extent that it is possible to operate a short wave telegraph transmitter as shown in the accompanying -drawing and aY short wave telephone receiver ysimultaneously with negligible interference in the telephone channel from the telegraph channel.

.. Having now particularly described and ascertained thenature of my said invention and in -what manner the same is to be performed, I declare that What I claim is:-

1. In combination a high frequency oscillatory rtube, a source of anode-potential for said tube, an -absorber tube having its anode to cathode impedance connected in shunt with said source, a source of alternating current, a circuit for supplying current from said alternating current source to a rectifier arrangement, a potentiometer .resistance shunted by a smoothing unit connected to the output of said rectifier arrangement, a connection between a point on said potentiometer 'and the control grid of the absorber tube, and a connection between a point on said potentiometer and the control grid of the high frequency oscillatory tube.

2. Transmitting means comprising, a thermionic oscillation generator having control grid and anode electrodes connected in oscillation producing circuits, and a source of energizing potentials connected with the anode of said tube and means for keying the oscillations produced comprising, a thermionic rectifier having input Aelectrodes connected by Way of a switch to a sourceof undulatory energy and output electrodes connected by way of a potentiometer to the control grid of said oscillation generator in such a manner that when said key is in one position said oscillation generator is biased to cut-off and in the other position said oscillation generator is operative, and means for insuring a constant drain on said source of potentials connected with the anode of said oscillation generator including, an additional thermionic tube having its anode connected to the rst named source of potentials and its control grid connected to a point on said potentiometer such that when said oscillation generator is biased to cut-off said additional tube is conductive, and vice versa.

3. Signalling means comprising, a thermionic relay tube having input and output circuits oper.

ative at carrier frequencies, saidoutput circuits being energized by a source ofv direct current potentials, means for keying the oscillations in said tube comprising a thermionic rectifier having an input circuit connected by way of a switch to a source of energizing energy and output electrodes connected by way of a resistance to the control grid of said tube, whereby the operativeness of said tube is determined by the open or closed postion of said switch, and means for insuring a constant drain of energy on said direct current source comprising an additional thermionic tube havingv its anode to cathode impedance connected in parallel with said source and its control grid connected to a point on rsaid resistance. Y e

POWELL BEST. 

